A method for preparing a magnetic recording medium containing the steps of coating a magnetic coating composition on a nonmagnetic support, sufficiently drying it and then treating it with calendering treatment is known. In this case, the binder used therein includes cellulose derivatives, a copolymer of vinyl chloride and vinyl acetate, polyurethane resins acrylic resins, a copolymer of acrylic acid or methacrylic acid with their esters, a copolymer of vinylidene chloride, synthetic rubbers and polyesters, which are used alone or in combination. It is also known that nitrocellulose is used as a binder and is used in combination with polyol and polyisocyanate to harden a coated film.
A conventional method for preparing a magnetic tape comprises forming a magnetic layer, sufficiently drying it, winding it up, and after a while providing calendering treatment thereon. Another conventional method comprises forming a magnetic layer, drying it completely, and then providing calendering treatment thereon to improve surface properties.
However, if calendering treatment is provided after a magnetic layer is completely dried, as described above, reaction for hardening the magnetic layer has proceeded, and in many cases, surface molding is difficult and thus satisfactory surface properties cannot be obtained. This tendency is prominent particularly when a rapidly hardenable binder is used. Accordingly, high temperature and high pressure are necessary for calendering treatment. But there is a problem that as the temperature and the pressure of a calendering roll is increased, the life of the calendering roll becomes decreased and the rate of calendering treatment also becomes decreased.
In order to solve the above problem, it is proposed that while controlling the amount of residual solvent in a magnetic layer, calendering treatment is provided on the magnetic layer containing an appropriate amount of the solvent as disclosed in JP-A-57-143737 and 61-261819 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). Specifically, in JP-A-57-143737 calendering treatment is provided on a magnetic layer having residual solvent in an amount of from 0.3 to 3 wt % based on a coated layer, and in JP-A-61-261819 calendering treatment is provided on a magnetic layer in an amount of from 20 to 150 .mu.l/m.sup.2.
JP-A-57-143737 and U.S. Pat. 4,391,851 disclose that calendering treatment is provided using nitrocellulose as a binder and adjusting the residual amount of solvent contained in the magnetic layer. The above method is suitable when a smoothing treatment by calendering is continuously conducted. But as nitrocellulose is used as a binder in that method, several serious problems arise. Namely, there is a high risk of burning and explosion and it is thus difficult to handle nitrocellulose. Further, an extremely large investment for security facilities must be made.
Similar to the above, JP-A-61-261819 also discloses that calendering treatment is conducted while controlling the residual amount of solvent contained in the magnetic layer. However, that method also has similar problems as above, because the method uses similar binders as above and conventional binders are used, and furthermore, satisfactory smoothness suitable for higher density recording cannot be attained by that method.
Recently, higher image qualities have been demanded for a magnetic recording medium. To meet the above requirements, it is necessary that the surface of a magnetic layer should be contacted as closely as possible with a video head or an audio head and surface smoothness should be, therefore, improved. Furthermore, dispersibility of ferromagnetic particles should strikingly be improved.
On the other hand, as the surface of a magnetic layer becomes smoother, the friction in a running system of a video tape recorder increases, running tension also increases, and, therefore, better running durabilities under harsh conditions are more and more required.